Method of locating a fault in a power distribution system using a direct current signal of a distributed resource modulated by an alternating current signal

ABSTRACT

The present invention provides a method to ensure that distributed resources of a power distribution system remain connected to the circuitry of the power distribution system when a fault occurs at a distributed resource node to assist in identifying the location of the fault by continuing to inject a direct current (DC) signal from the controllable voltage source converter of at least one DC voltage distributed resource into the distribution system and modulating an alternating current (AC) signal on top of the direct current (DC) signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This invention claims priority to currently pending U.S. patentapplication Ser. No. 14/166,467 filed on Jan. 28, 2014 and entitled,“Voltage Profile Based Fault Location Identification System and Methodof Use”, which claims priority to U.S. Provisional Patent ApplicationNo. 61/757,507 filed on Jan. 28, 2013 and entitled, “Voltage ProfileBased Fault Identification”.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under Grant No.EEC0812121 awarded by the National Science Foundation. The Governmenthas certain rights in the invention.

FIELD OF INVENTION

This invention relates to a voltage profile based fault locationidentification system for use in a power distribution system. Thevoltage profile based fault location identification system includespower electronic converters and employs a short circuit limiting faultcurrent.

BACKGROUND OF INVENTION

The IEEE interconnection standard recommends that the distributedresources (DRs) of a power distribution system be disconnected from thepower distribution system when the voltage level falls below arecommended threshold to ensure that the distributed resources do notinject power onto the main power grid of the power distribution system.The IEEE interconnection standard is additionally based on the fact thatas the voltage level of the distribution system drops, the distributedresource's voltage reference from the substation may no longer beavailable or may no longer be accurate.

Identifying the location of a fault in a traditional power system is achallenging task. Electric power only flows in one direction, i.e. fromthe substation to the various loads. Therefore, when a severe shortcircuit fault occurs in a distribution system, there is an associatedcurrent rise and accompanying voltage sag near the faulted node whichextends to every node that is downstream of the faulted node. The faultprotection system of a power distribution system currently known in theart responds to the short circuit fault by isolating the assumed faultednodes and all the downstream nodes of the actual faulted node.

In a power distribution system containing distributed resources, mostfault location technologies known in the art ignore the presence of thedistributed resources by assuming either low distributed resourcepenetration or no power injection from the distributed resources duringa fault situation. While there are additional fault locationtechnologies known in the art that do consider the presence ofdistributed resources, these technologies do not consider a currentlimited system when a fault situation does occur.

Accordingly, what is needed in the art is a system and method for faultlocation identification in a power distribution system that addressesthe presence of distributed resources and provides a current limitedsystem when a fault occurs.

SUMMARY OF THE INVENTION

The present invention provides a method to ensure that distributedresources remain connected to the circuit to assist in the faultlocation by continuing to inject current in the distribution system. Thesystem contains a plurality of power electronic based converters whichconvert local direct current (DC) of the distributed resources (DRs) tothe power grid alternating current (AC). These converters also have theability to limit the current in the system when a fault occurs; hence,protecting the system equipment against high fault currents.

In one embodiment of the present invention, a method of identifying thelocation of a fault in a power distribution system is provided. In thepresent invention, the power distribution system includes a plurality ofdistributed resources, and the method includes, injecting, by one ormore of the plurality of distributed resources, a current into the powerdistribution system, generating a voltage profile resulting from theinjection of current by the one or more distributed resources andanalyzing the voltage profile to identify the location of the fault inthe power distribution system.

In an additional embodiment, the present invention provides a system forlocating a fault in a power distribution system. The system includes apower distribution system including a plurality of distributed resourcescoupled to the power distribution system, one or more of the pluralityof distributed resources comprising a controllable voltage sourceconverter configured to inject a current into the power distributionsystem. The system further includes, a voltage profile generatorconfigured for generating a voltage profile resulting from the injectionof current by the one or more distributed resources and an analyzerconfigured for analyzing the voltage profile to identify the location ofthe fault in the power distribution system.

The invention accordingly comprises the features of construction,combination of elements, and arrangement of parts that will beexemplified in the disclosure set forth hereinafter and the scope of theinvention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made tothe following detailed description, taken in connection with theaccompanying drawings, in which:

FIG. 1 is voltage profile for a fault at node 2 and node 6 in atraditional distribution system, wherein the system containing nodistributed resources or they are disconnected during the fault inaccordance with an embodiment of the present invention.

FIG. 2 is voltage profile for a fault at node 2 and node 6 in adistribution system containing distributed resources in accordance withan embodiment of the present invention.

FIG. 3 is voltage profile for a fault at node 6 in a traditionaldistribution system and one containing distributed resources inaccordance with an embodiment of the present invention.

FIG. 4 is a schematic of a test system 11.9 kV (8 nodes) in accordancewith an embodiment of the present invention.

FIG. 5 is a block diagram illustrating a fault location identificationsystem in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Modern power distribution systems include distributed resources thatprovide local power generation and are connected to the powerdistribution system. Such local power generation sources includephotovoltaic (PV) systems, wind systems and microturbines. The numberand diversity of these local power generation sources is rapidlyincreasing. As the number of local power sources connected to anexisting power distribution system rises, the distribution system faultlocation methods currently known in the art have become increasinglyinadequate. Reasons for the increasing inadequacy of the current faultlocation methods include unreasonable cost of the system, systemcomplexity (mesh-like topology) and bidirectional power flow in thedistribution system that is not addressed by the current fault locationmethodologies.

The fault location identification system of the present invention takesadvantage of the existing topology of the power distribution system. Thefault location identification includes controllable voltage sourceconverters (VSCs) to assist in the location of the fault and alters thevoltage profile of the system in the presence of a fault condition.Utilizing controllable voltage source converters to locate the faultreduces miss-trips of the circuit breakers that result when relying onthe measured voltage when there is no electrical supply in a section ofthe distribution system as a result of a fault. The incorporation ofcontrollable voltage source converters in the fault locationidentification system will serve to boost the voltage of thedistribution system, locate the fault and provide rapid restoration ofthe distribution system.

Power distribution system currently known in the art do not employcontrollable voltage source converters and the voltage on a feederassociated with the distributed resource is expected to decrease as thedistance between the distributed resource and the power distributionsystem increases. In accordance with the present invention, if thedistributed resources in the power distribution system are allowed toinject power, the voltage profile of the system will change as shown inFIG. 1 for the prior art system and FIG. 2 for the fault locationidentification system of the present invention.

When a severe short circuit fault occurs in a distribution system, thereis an associated current rise and accompanying voltage sag near thefaulted node which extends to every node that is downstream of thefaulted node. The fault protection system of a power distribution systemcurrently known in the art responds to the short circuit fault byisolating the assumed faulted nodes and all the downstream nodes of theactual faulted node. As shown in the graph of FIG. 1, when a faultoccurs in a prior art fault location identification system, the voltageprofile resulting from the fault will provides very limited informationregarding the location of the fault. For example in the prior artsystem, when a fault occurs at node 2 100, in both the 1MVA 105 and12MVA 110 cases, the distributed resources at nodes 2 100 through 8 125will be disconnected from the power distribution system by the faultprotection system of the power distribution system. Similarly, a faultat node 6 130 will result in the distributed resources at nodes 6 130through 8 125 being isolated from the rest of the system.

In contrast, as shown in the graph of FIG. 2, the voltage profileresulting from a fault in accordance with the fault locationidentification system of the present invention identifies the fault ateither node 2 200 or node 6 205. For example, when a fault occurs atnode 2 200, it is seen that the voltage level 210 drops at node 2 200,but the voltage level at nodes 3 220 through 8 225 is maintained by theuse of the controlled voltage source converters at each of thedistributed resources associated with each of nodes 3 220 through 8 225.Additionally, when a fault occurs at node 6 205, it is seen that thevoltage level 215 drops at node 6 205, but the voltage level at nodes 7230 through 8 225 is maintained by the use of the controlled voltagesource converters at each of the distributed resources associated witheach of nodes 7 230 through 8 225, As such, the voltage profile providedby the fault location identification system of the present inventionresults from the injection of current from all the distributed resourceat each of the nodes in the system using the controllable voltage sourceconverters.

FIG. 3 illustrates the difference in the voltage profiles of a prior artfault location system and the fault location identification systememploying multiple distributed resource with controllable voltage sourceconverters achieved by the present invention. As shown, when a faultoccurs at node 6 310 in the prior art system, the voltage level drops atnodes 6 310 through 8 315, in contrast, with the fault locationidentification system of the present invention, when a fault occurs atnode 6 310, the voltage level at node 7 320 through 8 315 is maintainedby the controllable voltage source converters of the distributedresources at these nodes. As such, the voltage profile for a distributedresource system utilizing the fault location identification system ofthe present invention clearly indicates the fault at node 6 310 in thesystem of the present invention, whereas the fault is not clearlyidentified in the prior art system.

FIG. 4 is a schematic illustrating an exemplary embodiment of thepresent invention that was used to generate the graphs shown in FIGS.1-3. The physical distance between two adjacent distributed resourcenodes will also determine if the faulty node can be located accurately.To get an accurate voltage profile for a power distribution system, itis necessary to measure the voltage at multiple nodes in the system.However, making measurements at the nodes is difficult in the prior artsystems because there are limited nodes at which a voltage measurementcan be performed. In contrast, in the fault location identificationsystem of the present invention which includes a plurality ofdistributed resources and a controllable voltage source converterassociated with each of the distributed resources, each controllablevoltage source converters can serve as a measurement unit at whichvoltage measurement can be performed and used to generate an accuratevoltage profile. As shown in FIG. 4, each of the nodes 1-8 arepositioned at varying distances from the distribution grid. It is knownthat the voltage level naturally decreases the farther the node is awayfrom the distribution grid. In the present invention, the controllablevoltage source converters at each of the nodes are used to injectcurrent into the power distribution system thereby providing the systemwith a means for measuring the voltage level at each of the nodes.Knowing the distances between the nodes will improve the accuracy of thefault location identification system of the present invention.

The voltage profile of the power distribution system changes when afault occurs in the system and the location of the observed voltage dropin the voltage profile is closely related to the location of the fault.Furthermore, most distributed resources provide DC voltage. Therefore,in a DC system, controllable voltage source converters can modulate anAC signal on top of the DC signal and that modulated AC signal profilemay be used to locate the fault similarly to the AC system.

In an exemplary embodiment, as illustrated with reference to FIG. 5, thefault location identification system of the present invention includes apower distribution system 400 and a plurality of distributed resources405, 410, 415, 420 coupled to the power distribution system 400. One ormore of the plurality of distributed resources 405, 410, 415, 420comprising a controllable voltage source converter 425, 430, 435, 440configured to inject a current into the power distribution system 400.The power distribution system 400 further includes, a voltage profilegenerator 450 configured for generating a voltage profile resulting fromthe injection of current by the one or more distributed resources 405,410, 415, 420 and an analyzer 445 configured for analyzing the voltageprofile to identify the location of the fault in the power distributionsystem 400.

It will be seen that the advantages set forth above, and those madeapparent from the foregoing description, are efficiently attained andsince certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatters contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention that, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A method of identifying the location of a faultin a power distribution system, the method comprising: in response to afault located at one of a plurality of distributed resources on one ormore feeders of a power distribution system comprising a distributiongrid and wherein a distance of each of the plurality of distributedresources relative to the distribution grid is known, generating acurrent to be injected into the power distribution system by acontrollable voltage source converter of one or more of the plurality ofdistributed resources not located at the fault on the feeder, whereinthe one or more of the plurality of distributed resources not located atthe fault on the feeder is a DC voltage distributed resource providing aDC signal and injecting, by the one or more of the plurality of DCvoltage distributed resources not located at the fault on the feeder,the current into the power distribution system; modulating, by thecontrollable voltage source converter of the DC voltage distributedresource, an alternating current (AC) signal on top of the directcurrent (DC) signal of the DC voltage distributed resource; measuring avoltage level at each of the plurality of distributed resourcesresulting from the current injected into the power distribution systemby each of the one or more distributed resources not located at thefault on the feeder; generating a voltage profile from the voltagemeasurements at each of the one or more distributed resources, whereinthe voltage profile comprises the voltage level at each of the pluralityof distributed resources relative to the distance of each of theplurality of distributed resources from the distribution grid of thepower distribution system; and analyzing the voltage profile to identifythe location of the fault in the power distribution system, whereinanalyzing the voltage profile further comprises identifying a voltagedrop at one of the plurality of distributed resources located at a firstdistance from the distribution grid and a corresponding voltage rise atone or more of the plurality of distributed resources located a seconddistance from the distribution grid, wherein the second distance isgreater than the first distance.
 2. The method of claim 1, furthercomprising maintaining an electrical connection between a firstdistributed resource of the plurality of distributed resources and thepower distribution system when the fault is at the location of the firstdistributed resource.
 3. The method of claim 1, further comprising,isolating the identified fault in the power distribution system.
 4. Themethod of claim 1, wherein at least one of the plurality of distributedresources is selected from the group consisting of a wind turbine,microturbine, solar panels and cogeneration resources.